The invention is particularly, but not exclusively, concerned with fastening devices of the type described in UK patent publication GB-A-2245681 in which an annular metal cylinder body is provided with an annular cavity in one end face thereof to receive an annular metal piston the radially inner and outer side walls of which terminate as tapering annular webs which together define an annular recess between them and exhibit such flexibility under hydraulic pressure to seal against the cavity walls whilst the piston is displaced axially along the cavity by hydraulic pressure therein. That device distinguishes from conventional hydraulic nuts by the form of sealing which obviates the necessity for materials which are unable to withstand prolonged exposure to elevated temperatures.
That specification is concerned with a hydraulic nut in which the annular metal cylinder body comprises an internally threaded nut body adapted to be threaded onto an elongated bolt or stud protruding from apparatus and the annular metal piston comprises a load ring adapted to bear against the apparatus as the piston and cylinder are separated by hydraulic pressure to induce a tensile stress in the bolt or stud. The piston is part of a piston body which includes a radially outwardly extending flange that faces the part of the cylinder body radially outwardly of the cavity such that shims inserted between said flange and body part maintain the axial juxtaposition, and thus the axial loading, after the hydraulic pressure is removed.
Although the specification, which relates principally to the sealing arrangement, describes it in relation to such a hydraulic nut in which tension loading is retained by shims, it will be appreciated that it is also conventional to maintain the relative positions of piston and cylinder body under load by means of a locking ring internally threaded to be supported by co-operating threads on the periphery of the piston or cylinder body and displaceable axially into abutment with the other prior to venting of hydraulic pressure, whereby when hydraulic pressure is removed the load is then transmitted from one body to the other by way of the abutment, ring and threaded support.
A particular use for such a hydraulic nut having the above described sealing arrangement is in a turbine casing where the nut may have to retain tensile loading from casing bolts for many years at elevated temperatures before removal by applying hydraulic fluid pressure, removing the physical retention, that is, displacing the locking ring, and reducing the hydraulic pressure to relieve the tensile loading in the bolt.
Typically in such on application, the spacing pitch of casing bolts is limited by the dimensions of the hydraulic nuts and it is desirable to minimise the radial dimensions. It is convenient to have the locking ring to surround and be supported by the piston body and be displaced axially into abutment with an end face portion of the cylinder body radially outwardly of the annular cavity so that the radial dimensions of the hydraulic nut are effectively defined by the radial dimensions of the annular cavity, dictated by fluid pressure and axial force required to be exerted, and by the radial dimensions of the cylinder body forming walls for the cavity, in withstanding chamber pressure acting radially thereon and the axial tensile loading supported after removal of pressure.
Hitherto, the radial thickness of particularly the portion of the cylinder body outwardly of the cavity has been chosen such that any distortion or deflection of the wall due to hydraulic pressure is within its elastic limits so that the wall returns to its original configuration with respect to the piston and seal arrangement when hydraulic pressure is removed. It will be appreciated that where traditional elastomeric sealing arrangements have been used the requirements for return to the precise configuration are less stringent that with the metal web scaling of the above described specification.
However it has been found that notwithstanding the choice of dimensions and materials of the cylinder body within such design limits that any hydraulic pressure induced cylinder body deflection relaxes to within the sealing ability of the metal piston webs, when the cylinder body is caused to support the axial load by way of a locking ring on the piston, there is sealing deficiency that impedes, or even prevents, subsequent pressurisation of the cavity.
It will be appreciated that the head of a bolt is directly analogous to a stud or bolt shank having a nut threadedly secured thereto in respect of applying axial forces and that the aforementioned discussion applies equally to a bolt head having an integral hydraulic tensioning arrangement as it does to a separately attached nut. In this specification the term "hydraulic tensioning arrangement" is, unless otherwise qualified, intended to refer to either nut or bolt form.
Insofar so such sealing deficiency results from forces acting on the body it must be considered to exist, albeit to a lesser degree, with any form of sealing arrangement between the piston and cylinder body and thus accommodating the effect of such forces on the body is independent of the precise form of sealing and the piston and cylinder body structures that define a cavity.